Geology, Geochemistryand Ore-forming Process Of The Sijiaying Iron Deposit In The Eastern Hebei Province, North China Craton

The eastern Hebei Province, where numberous large-superlargemeta-sedimentary type of iron deposits (metamorphic banded iron formation) distributed, is located at the northern part of North China Craton. Among them, Sijiaying iron deposit is the largest. Great progresses have been achieved in exploration and hundreds of millions of tons of resources have been added in this iron deposit recently. However, the researches are still very poor. Sijiaying iron deposit is hosted in Neoarchean luanxian Group which was metamorphosed to epidote-amphibolite facies,consisting mainly of biotite leptynite. Faults and small-scale magma intrusions are well developed in this iron deposit. Few magnetite-type of high-grade iron ore (TFe>50%) and hematite-type of iron ore formed after a series of geological processes. On the basis of field geological survey and indoor petrography, ore microscopy, using by these technical means of electron microprobe, inductively coupled plasma mass spectrometry, SHRIMP,isotope mass spectrometry and mircothermometry, doing some studies included mineralogy, major-trace element geochemistry, stable isotope, fluid inclusion, geochronology, and etc., comparing geological characteristics with those of other typical iron deposits in the eastern Hebei Province, to understand the ore-forming age, geotectonic background and source of metallogenic material, to discuss ore-forming progress, then to build sedimentary metallogenic model and hydrothermal alteration model, in order to further summarize mineralization mechanism and metallogenic regularity of iron deposits in the eastern Hebei Province. The thesis has proposed several points as follows.(1) The ore types mainly are maganetite-quartzite, actinolite-maganetite-quartzite and amphibole-maganetite-quartzite.The metallic minerals are magnetite and hematite. The gangue minerals are quartz, actinolite, amphibole, chloriteand biotiteetc..The content ofmetallic mineralsis generally about40%.The ore structures mainly are striped, banded, massive and breccia; ore textures include granulous crystalloblastic, flaky-granulous crystalloblastic, metasomatic, inclusion-crystalloblastic and crush texture.(2)According to field geological characteristics, ore fabric, mineral paragenesis and previous studies, dividing the sedimentary and late-reform mineralization into five stages:depositional stage, epidote-amphibolite facies metamorphic stage, folding and deformation stage, hydrothermal alteration stage, uplifting and oxidizing stage. The main ore-forming stage is depositional stage, a small amount of high-grade iron ores and hematite were formed during hydrothermal alteration stage and uplifting and oxidizing stagerespectively.(3) Different types of iron ores have similar characteristics of major and trace elements. The main chemical constituents of iron ores are SiO2, Fe2O3and FeO. The concentration of REE in iron ore is low, Post Archean Australia Shale (PAAS)-normalized rare earth elements (REE+Y) profiles for the iron ores displayed depletion of light REE relative to high REE. All samples show distinct positive anomalies of Eu and slightly positive anomalies of Y, no obvious anomalies of Ce, Pr and Gd, high Y/Ho ratio. Moreover, low content of High Field Strength Element (HFSE) indicate that minor continental detritus input. The typical characteristics of the REYs profiles resemble that of the mixture of high-temperature hydrothermal fluid and seawater, suggesting iron may derived from submarine high temperature hydrothermal fluids mixed sea water.(4) Plagioclases occurred in biotiteleptynite are andesine and oligoclase, indicating the protolith of biotite leptynite were intermediate volcanic rocks while biotites are lepidomelane, reflecting the protolith of biotite leptynite were Fe-rich rocks.The geochemistry characteristics of biotiteleptynite are iron-rich, calcium-poor, the Al2O3/TiO2ratio range from28to40, reflecting of high aluminum and titanium, which indicating of crustal magma affinity. In the primitive mantle-normalized spider diagram, showing obvious Rb, K, La enrichment and high field strength elements Zr and Hf, Nb, Sr, P and Ti negative anomalies, showing a significant crustal magma characteristics. The protolith of biotite leptynite was dacite,On the Rb vs. Y+Nb and Nb vs.Y tectonic discrimination diagrams, all the samples were ploted in the fields of volcanic-arc granite or volcanic-arc granites plus syn-collisional granite, reflectingformed in a back-arc basin tectonic setting.(5) The zircon SHRIMP U-Pb ages of interlayered biotite leptynite and intrusive gneissic monzonite granite were2539±6Maand2529±5Ma respectively, indicating the formation time of Sijiaying iron deposits was between2529Ma and2539Ma.(6)The metallogenic materialsprobably originated from oceanic crust following hydrothermal alteration by deeppercolating seawater, transformingmeta-sedimentary type of iron deposit by the metamorphism caused by diaper of "Qian’an" gneiss which occurred in about2500Ma.The peak metamorphism was epidote-amphibolite face, metamorphic fluids belong to medium temperature(352-560℃) and low pressure(0.11～0.20GPa) fluids, metamorphism and deformation unaltered the grade of iron ore. A small amount of high-grade iron orebodies were formed by hydrothermal replacement of striped maganetite-quartzite along the faults and shear zones.Hydrothermal fluids belong to NaCl+H2O system and temperature concentrated in153～212℃. The fluid mainly originated from the meteoric water.